Which alkaline substance will produce the least amount of sludge when treating water?

Caustic soda, or sodium hydroxide, is known for its high solubility in water. This characteristic allows it to effectively raise the pH of water without significantly contributing to the formation of insoluble compounds that would settle as sludge. When caustic soda is added to wastewater, it primarily reacts with free acids to neutralize them and does not precipitate as much solid material compared to other alkaline substances.

In contrast, hydrated lime, magnesium hydroxide, and quicklime all tend to produce more sludge as they react with impurities in water. For example, both hydrated lime and quicklime can generate calcium carbonate precipitates when they react with carbonate ions in the water, leading to increased sludge production. Magnesium hydroxide can also lead to the formation of magnesium salts that contribute to sludge. Therefore, caustic soda is the most efficient in minimizing sludge formation during the treatment process.